As the accumulation of extensive theoretical and experimental evidence from investigation of the EDM process indicates, ultra-fine finish EDM results can be obtained by controlling each individual machining (discharge) pulse which is repetitively passed through the EDM gap to limit its duration or so-called on-time .tau.on in the range between 0.01 and 1 microsecond, preferably in the order of 10 nanoseconds. The successive discharges should be discrete and should be time-spaced by an "off" time .tau.off or pulse interval likewise precisely controlled at a suitable value, preferably not greater than five or ten times longer than the on-time, so that the application of a predetermined amount of energy from the power supply is strictly confined in each pulse. A power-supply system which would be so designed is also desired to be capable of wide selection of on-time and off-time of required machining pulses precisely independent from each other.
Attention is also drawn to the preference in some important areas of the use of a certain mode of machining pulses, development of which was initiated more than a decade ago, even extending back to incipient EDM history (cf. Japanese patent specifications No. 39-20494 published 19 Sept. 1964 and No. 44-8317 published 18 Apr. 1969) but whose particular advantages have increasingly been recognized in the recent years. Thus, machining pulses may advantageously be applied in the form of intermittently interrupted elementary pulses or successive, time-spaced trains of elementary pulses. The term "elementary pulse" is used to refer to a unit or minimum energy pulse whereby a unit or minimum volume of stock removal .eta. is assured. In that mode, each train has a preset duration Ton and thereby contains a preset number n of elementary pulses which represents essentially a total stock removal of the amount (n.times..eta.) and adjacent trains are separated by a time interval Toff. Thus, given an elementary pulse of on-time .tau.on and off-time .tau.off, a series of any desired "composite" pulses are obtainable by simply adjusting the train duration .tau.on or the number of elementary pulses in each train and the time interval .tau.off or the number of elementary pulses to be periodically interrupted. Here again, the unit pulse is desirably assigned a minimum on-time .tau.on and also a minimum off-time .tau.off of the same order.
Whichever, consecutive pulses or periodically interrupted elementary pulses, are used, each precisely time-controlled pulse of an extremely short on-time .tau.on and .tau.off is best formed in a power-supply system designed to "pulse" the DC output by means of an electronic switch having "on" and "off" states in response to the corresponding drive "on" and "off" signals. Note that the use of FET's (field-effect type transistors) or MOS's (metal-oxide semiconductor transistors) now allows such extremely rapid switching in the system. In the prior EDM art, however, no system or method which is designed to reliably provide or capable of truly satisfactorily furnishing EDM pulses of on-time and off-time as short as less than 1 microsecond precisely in accordance with corresponding drive signal pulses could be realized.